1. Field of the Invention
This invention relates to an apparatus for controlling a synchronous drive for direct-current motors, which apparatus provides two characteristics, that is, the self-starting characteristic which is inherent in D.C. motor and the constant speed characteristic which is inherent in a synchronous motor.
2. Description of the Prior Art
Conventional D.C. motors may obtain a great starting torque, but the number of revolutions per minute thereof varies with fluctuations of load. Also, conventional synchronous motors have both merits and demerits in that they may have good constant speed compared to the D.C. motors but they may not be able to produce a sufficient starting torque, to effect self-starting. On the other hand, conventional induction motors may eliminate those disadvantages as noted above with respect to the above-described two kinds of motors to some extent, but induction motors are not able to obtain sufficiently both the starting torque characteristic and constant speed characteristics, and their speed is unavoidably lowered due to slip.
Therefore, there has existed a need for rotary motors which can provide great starting torque and sufficient constant speed.
Methods heretofore proposed to fulfill the requirements as noted above are:
1. A METHOD FOR DRIVING A D.C. motor at a synchronous speed in which when a rotor of the motor has reached a predetermined number of revolutions per minute, a synchronous signal from a synchronized device, externally provided, is transmitted to the D.C. motor thereby causing it to rotate at a synchronous speed; and
2. A METHOD IN WHICH A SPECIFIC STARTING DEVICE IS EMPLOYED FOR A SYNCHRONOUS MOTOR.
However, in these methods, the construction of the motor and its accessory devices becomes complicated, or additional operation is needed at the time of start, requiring the change from a D.C. machine to a synchronous machine, automatic operation of which has been difficult.